Here's a sight for four eyes. Scientists say they've found evidence of a fourth eye in the fossil skulls of extinct monitor lizards — an ocular abundance found in no other known jawed vertebrate.
The discovery, published in the journal Current Biology, offers scientists a strange bloom on the vertebrate evolutionary tree.
Believe it or not, many fishes and frogs have what's known as a parietal eye, or a "third eye," in the middle of their foreheads that can sense light and is linked to the pineal gland, which produces melatonin, the hormone that helps regulate sleep patterns.
Many lizards and tuataras also have a light-sensitive "third eye," but scientists argue that it may have evolved from a separate organ right next to the pineal organ called the parapineal organ.
That third eye is typically rudimentary but still has the basic parts of an eye — connective tissue like the whites of our eyes as well as a very simple retina and lens.
"It's not a great eye, but it's a real eye," said study co-author Bhart-Anjan Bhullar, a paleontologist and developmental biologist at Yale University.
Such third eyes might be useful for absorbing slightly different wavelengths of sunlight, keeping the body's circadian rhythm in sync, signaling the reproductive season based on the time of year, or even helping an animal to orient itself in space.
"Deep on the stem of the mammal lineage, our ancestors also had a third eye on the top of the head," Bhullar said.
Over time, many vertebrate lineages independently lost this third eye.
But scientists think our early vertebrate ancestors also had a fourth eye, he added — one that was lost much earlier in our evolutionary history.
"All vertebrates including our ancestors ancestrally had four eyes," Bhullar said. "That's the basal state for all vertebrate animals."
The only vertebrate species today with four eyes is the jawless lamprey, he added, "because lampreys are some of the most primitive survivors of a lineage that diverged from other vertebrates earlier than anything else."
But years ago, lead author Krister Smith, a paleontologist at the Senckenberg Research Institute in Germany, found a fossil that led him to suspect that there might be reptiles who could have regained a fourth eye.
So the scientists went looking, trawling through museum specimens gathered in 1871 from Bridger Basin, Wyoming. They found fragmentary skull bones from two individuals belonging to an extinct monitor lizard species, Saniwa ensidens, that lived some 49 million years ago.
When they subjected the fossils to CT scans, they found that there were two cavities right where the third and fourth eye should be – and the fourth-eye cavity was shaped as if an eye had indeed been present, with a hemispherical top leading into a stem, rather like a mushroom or a flower.
"It's sort of regained that ancestral four-eyed condition after hundreds of millions of years of having three eyes," Bhullar said.
Why a fourth eye would have reemerged in this species, and what evolutionary benefits it provided them, remains a mystery.
"They probably have a number of different functions that we don't know about," he said.
For an animal that likely had to regulate its body temperature by spending time in the sun or avoiding it, having two separate organs staring up at the sky might have been very useful, Bhullar said.
Still, even among the jawless lampreys that live today, he added, the purpose of a fourth eye is poorly understood.
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